1. Field of the Invention
The present invention relates to piezoelectric ceramic compositions and piezoelectric ceramic devices using the compositions. Particularly, the present invention relates to piezoelectric ceramic compositions which are useful as materials for, for example, piezoelectric ceramic filters, piezoelectric ceramic oscillators, piezoelectric ceramic resonators and other piezoelectric ceramic devices, and to piezoelectric ceramic devices using the compositions.
2. Description of the Related Art
Piezoelectric ceramic compositions mainly containing lead titanate zirconate (Pb(TixZr1xe2x88x92x)O3) or lead titanate (PbTiO3) have been widely used as piezoelectric ceramic compositions for piezoelectric ceramic filters, piezoelectric ceramic oscillators, piezoelectric ceramic resonators and other piezoelectric ceramic devices. However, such piezoelectric ceramic compositions mainly containing lead titanate zirconate or lead titanate inevitably contain large amounts of lead as a component, and lead oxide evaporates in the production of the piezoelectric ceramics to deteriorate the uniformity of characteristics of the produced devices. To avoid deteriorated uniformity of characteristics of the produced devices due to vaporization of lead oxide in the production step, piezoelectric ceramic compositions containing no or small amounts of lead are preferred.
In contrast, piezoelectric ceramic compositions mainly containing SrBi2Nb2O9 or other layered bismuth compounds include no lead oxides in the composition and do not invite the above problem.
However, piezoelectric ceramic compositions mainly containing SrBi2Nb2O9 or other layered bismuth compounds must be fired at high temperatures of 1250xc2x0 C. or more to obtain piezoelectric ceramics exhibiting a practicable electromechanical coefficient kt (10% or more). Expensive kilns that can perform firing at such high temperatures are therefore required. In piezoelectric ceramic filters or other piezoelectric ceramic devices having inner electrodes, electrode materials should have a melting point higher than the firing temperature. Such piezoelectric ceramic devices therefore require expensive electrode materials such as platinum or palladium, which invites increased production costs.
Accordingly, it is a main object of the invention to provide piezoelectric ceramic compositions mainly containing SrBi2Nb2O9 which can be fired at lower temperatures, exhibit a practicable electromechanical coefficient kt even when fired at firing temperatures of 1100xc2x0 C. or less, and which are useful as materials for, for example, piezoelectric ceramic filters, piezoelectric ceramic oscillators, piezoelectric ceramic resonators and other piezoelectric ceramic devices, and to provide piezoelectric ceramic devices using the compositions.
The invention provides, in one aspect, a piezoelectric ceramic composition mainly containing a component corresponding to the formula SrBi2Nb2O9. The composition includes at least Si in a proportion of more than 0 mol and equal to or less than about 0.3 mol relative to 1 mol of Bi in the main component shown by the formula. The invented piezoelectric ceramic composition may include, for example, a component shown by the formula SrBi2(Nb1xe2x88x92ySiy)2O9, wherein y is more than 0 and equal to or less than 0.3.
The invented piezoelectric ceramic composition may further include a divalent metallic element other than Sr in a proportion of more than 0 mol and equal to or less than about 0.25 mol relative to 1 mol of Bi. In this case, the divalent metallic element other than Sr may be at least one selected from Mg, Ca and Ba.
The invented piezoelectric ceramic composition may further include a trivalent metallic element other than Bi in a proportion of more than 0 mol and equal to or less than about 0.15 mol relative to 1 mol of Bi. In this case, the trivalent metallic element other than Bi may be at least one selected from the group consisting of La, Ce, Pr, Nd, Sm, Gd, Dy, Er, Yb, Sc and Y.
Preferably, the invented piezoelectric ceramic composition may further include manganese in a proportion of more than 0% by weight and equal to or less than about 1.0% by weight in terms of MnCO3.
In another aspect, the invention provides a piezoelectric ceramic device. The piezoelectric ceramic device includes a piezoelectric ceramic composed of the piezoelectric ceramic composition and electrodes formed on the piezoelectric ceramic.
The invented piezoelectric ceramic composition should include Si in a proportion of more than 0 mol and equal to or less than about 0.3 mol relative to 1 mol of Bi. If the content of Si exceeds this range, the electromechanical coefficient kt is decreased and a practically sufficient electromechanical coefficient cannot be significantly obtained.
The invention is also effective for piezoelectric ceramic compositions each containing Mg, Ca, B or another divalent metallic element, or La, Ce, Pr, Nd, Sm, Gd, Dy, Er, Yb, Sc, Y or another trivalent metallic element in the main component. Especially, when the main component includes at least one element selected from La, Ce, Pr, Nd, Sm, Gd, Dy, Er, Yb, Sc, and Y in a proportion of more than 0 mol and equal to or less than about 0.15 mol relative to 1 mol of Bi, the temperature-dependent factor of resonant frequency within a temperature range from xe2x88x9220xc2x0 C. to 80xc2x0 C., i.e., frTC shown by the following equation, is decreased, although the reason of this phenomenon is not known.
frTC=(fr(max)-fr(min))/(fr(20xc2x0 C.)xc3x97100) 
wherein fr(max) is the maximum resonant frequency within the temperature range from xe2x88x9220xc2x0 C. to 80xc2x0 C.; fr(min) is the minimum resonant frequency within the temperature range from xe2x88x9220xc2x0 C. to 80xc2x0 C.; and fr(20xc2x0 C.) is the resonant frequency at 20xc2x0 C.
Accordingly, resulting piezoelectric ceramic compositions can be fired at temperatures of 1100xc2x0 C. or less, have a practicable electromechanical coefficient and have a low temperature-dependent factor of resonant frequency frTC. These piezoelectric ceramic compositions are useful as, for example, materials for piezoelectric ceramic resonators and other piezoelectric ceramic devices. In this case, the main component should preferably include at least one selected from La, Ce, Pr, Nd, Sm, Gd, Dy, Er, Yb, Sc and Y in a proportion of more than 0 mol and equal to or less than about 0.15 mol relative to 1 mol of Bi. If the content of these elements exceeds the above range, the electromechanical coefficient kt is decreased and the incorporation of these elements does not exhibit satisfactory advantages, as compared with compositions which contain none of the above elements.
The present inventors also found that the electromechanical coefficient can be improved by incorporating manganese in a proportion of about 1.0% by weight or less in terms of MnCO3 into a piezoelectric ceramic composition mainly containing SrBi2Nb2O9 (Japanese Patent Application No. 10-156648). The present invention is also effective for this type of piezoelectric ceramic compositions. The resulting piezoelectric ceramic compositions can be fired at temperatures of 1100xc2x0 C. or less, have a practicable electromechanical coefficient, and are useful as materials for piezoelectric ceramics.
Further objects, features, and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.